1. Field of the Invention
The present invention relates to a filter apparatus including at least one pass band and multiple attenuation bands.
2. Description of the Related Art
In a filter apparatus described in Japanese Unexamined Patent Application Publication No. 2012-257050, a surface acoustic wave (SAW) resonator is combined with an LC filter. The SAW resonator provides a trap.
International Publication No. WO 2007/097186 discloses a surface acoustic wave apparatus using a piezoelectric layer made of LiNbO3. It is indicated that a higher order mode of shear horizontal (SH) waves appear in the pass band as a spurious in the surface acoustic wave apparatus using the piezoelectric layer made of LiNbO3. Accordingly, an increase in loss occurs.
When it is necessary to provide traps in two frequency bands using the SAW resonator, for example, as described in Japanese Unexamined Patent Application Publication No. 2012-257050, it is necessary for a trap circuit to be cascade-connected. Accordingly, the loss is increased and the size of the filter apparatus is liable to be increased.
On the other hand, as described in International Publication No. WO2007/097186, the higher order mode of the SH waves has been known to be the spurious causing the increase in loss. Accordingly, it is considered that the higher order mode of the SH waves should be suppressed.